Project Summary The alarming rise of antibiotic resistant strains of bacteria have outpaced the development of new antibiotics capable of treating them. There is obvious concern over the transmission and spread of these resistant strains within the human population, but there is now considerable evidence of their transmission from infected animals to humans. The broad ramifications of this troubling public health scenario underscore the urgent need for a new therapeutic strategy that extends beyond the use of conventional antibiotics and their limitations in the face of rapid bacterial mutation that give rise to resistant strains. A particularly promising therapeutic approach to provide this unmet need is mobilizing innate immune responses with a safe and effective immune stimulatory molecule. Such ?host-directed immunotherapy? not only will invoke the body's inherent first line of defense against infection, but will also contribute few/no mutational pressures since this therapy is not imposed directly on the bacteria in the manner of antibiotics. The potential of host-directed immunotherapy against resistant bacterial and viral infections in mice has been demonstrated by the administration of our lead immune stimulatory candidate EP67, a response selective decapeptide agonist of the C5a receptor (C5aR/CD88) derived from the biologically active C-terminal region of human complement component C5a (C5a65-74). Against this backdrop, the long-term goal is to establish EP67 (or an improved analogue) as an FDA-approved therapeutic for the treatment of antibiotic-resistant infections via the safe and effective induction of host innate immunity through the C5a/C5aR network on antigen presenting cells. The principal objective of this R01 project, which is an important step toward this goal, is to move the lead candidate EP67 (or analogue) along an IND-leading pathway that strengthens such designation by the FDA. This objective will be approached by the following specific aims: 1) Down-select from our lead candidate EP67 by refining/enhancing the structure-function attributes of EP67 from a series of analogues with residue substitutions for Pro at position 7; 2) Demonstrate therapeutic efficacy of EP67/analogue in inducing host- innate immune responses against local and systemic methicillin-resistant Staphylococcus aureus (MRSA) infections in the human-relevant porcine model; and 3) Initiate safety/toxicity/pharmacokinetic studies of EP67/analogue in industry standard murine models. The team assembled for this project will bring the technological, clinical, regulatory, and product development expertise necessary for an IND application and sustainability through the FDA regulatory process leading to clinical trials. This will have a positive impact in the field by bringing to the forefront the concept of host-directed immunotherapy as a safe and effective strategy for treating antibiotic resistant infections within the therapeutic framework of ?One Health?.